Control of asphaltic oil hydrode-sulfurization process

ABSTRACT

ASPHALT-CONTAINING OILS ARE HYDRODESULFURIZED WITHOUT EXCESSIVELY CONVERTING THE ASPHALTENES AND RESINS PRESENT BY CONTROLLING THE EXTENT OF DESULFURIZATION OF THE ASPHALTIC PORTION OF THE OIL IN RESPONSE TO THE QUANTITY OF AROMATICS PRESENT. AS THE ASPHALTIC OIL PASSES THROUGH A HYDRODESULFURIZATION ZONE, THE AROMATIC CONTENT OF THE OIL INCREASES DUE TO IN SITU PRODCTION OF AROMATICS. THE OIL IS WITH-   DRAWN FROM THE HYDRODESULFURIZATION ZONE WHEN THE AROMATIC CONTENT NO LONGER INCREASES.

June 4, 1974 MC E'TAL 3,814,68}

CONTROL OF ASPHALTIC OIL HYDRODESULFURIZATION PROCESS Filed Dec. 8, 19714 Sheets-Sheet 1 I O 01 cu j 2 a 5 Q 5 LI.

MAKE-UP HYDROGEN June 4, 1974 J. MCKINNEY ET AL 3,814,681

CONTROL OF ASPHALTIC OIL HYDRODESULFURIZATION PROCESS 4 Sheets-Sheet 2Filed Dec. 8, 1971 750 AVERAGE REACTOR TEMPERATURE: F.

.5053 5 Q E; oowm FIG. 2

SATURATES mzommgome: +503 3 Emzthmzoo 6 .5653

I0 4O 5O 6O 7O I00 PERCENT DESULFURIZATION FIG. 3

June 4, 1974 J. D. MCKINNEY ET AL 3,814,681

CONTROL OF ASPHALTIC OIL HYDRODESULFURIZATION PROCESS Filed Dec.

FIG. 4

FIG. 6

VOL.9% 0F COMPONENTIN OVERHEAD o68 3883888 l 4 Sheet-Sheet 5 June 4,1974 J. McKm Y ETAL 3,814,681

CONTROL OF ASPHALTIC OIL HYDRODESULFURIZATION PROCESS Filed Dec. 8, 19714 Sheets-Sheet 4 424 FIG. 7

United States Patent 3,814,681 CONTROL OF ASPHALTIC OIL HYDRODE-SULFURIZATION PROCESS Joel D. McKinney and John A. Paraskos, Pittsburgh,Pa.,

assiguors to Gulf Research & Development Company, Pittsburgh, Pa.

Filed Dec. 8, 1971, Ser. No. 206,078 The portion of the term of thepatent subsequent to Nov. 27, 1990, has been disclaimed Int. Cl. Cg23/02 US. Cl. 208210 9 Claims ABSTRACT OF THE DISCLOSUREAsphalt-containing oils are hydrodesulfurized without excessivelyconverting the asphaltenes and resins present by controlling the extentof desulfurization of the asphaltic portion of the oil in response tothe quantity of aromatics present. -As the asphaltic oil passes througha hydrodesulfurization zone, the aromatic content of the oil increasesdue to in situ production of aromatics. The oil is withdrawn from thehydrodesulfurization zone when the aromatic content no longer increases.

This invention relates to a process for the production of crude orresidual oils having an especially low sulfur content. Moreparticularly, this invention relates to a hydrodesulfurization processfor producing an asphaltic fuel oil having a low sulfur content.

Various processes have been proposed for the desulfurization ofpetroleum hydrocarbons, and such processes have been highly effective inthe removal of sulfur from the relatively lighter distillate fractionsthat are obtained from the distillation of crude oil. However, crude oiland reduced crude oil fractions contain a heavy, residue fractionboiling above 1040 F. commonly referred to as asphalt, which renders theoil much more difiicult to desulfurize. Asphalt is a generally low-gradematerial that has been isolated from the crude oil and utilized in roadconstruction. In addition, it has been proposed to treat the asphalticfraction of crude oil in some manner so as to upgrade this material intoa more valuable product such as a fuel oil. However, asphalt, whenisolated, is highly viscous and is difficult to process over a catalyst.Moreover, it consists of large molecules of fused, aromatic rings andcontains the greatest amount of the total sulfur content of the fullrange crude on a relative basis. A further problem is that some of thesulfur content of the asphalt is tied up in the interior of the largemolecules thereof, rendering the asphalt especially difiicult todesulfurize. In addition, asphalt contains metals, principally nickeland vanadium, which readily deposit on desulfurization catalysts tendingto deactivate the catalyst. The cumulative effect of such factors isthat much more severe conditions in the form of higher temperatures andpressures are required as well as a different type of catalyst in orderto remove the sulfur from heavy, asphaltic oils as compared withdesulfurization of a distillate oil.

Sulfur is a major contributor to air pollution. Accordingly, certainmunicipalities, both local and foreign, have placed an upper limit onthe sulfur content of fuel oils. In the past, such locales have placed aone percent by weight sulfur limit on heavy fuel oils, but the trend hasbeen to a lower maximum sulfur content, such as 0.5 percent. Suchlimitations have provided a major impediment to the use of heavyasphaltic hydrocarbon oil fractions as fuel oil, since an asphaltic fueloil having an 0.5 percent sulfur content is not easily obtained fromsome high sulfur crudes merely by passing the crude oil over adesulfurization catalyst.

For reasons previously unappreciated, the sulfur conice tent of aparticular asphaltic crude or reduced oil could be reduced to onepercent sulfur in a hydrodesulfurization zone, but if the sulfur levelwas to be reduced below one percent, the desulfurization temperature hadto be increased to a point at which excessive hydrocracking to lowerboiling materials resulted. Thus, while it had been relatively easy toreduce the sulfur level of such residual oils from 4 to 1 percent byweight of sulfur without incurring excessive cracking of the feed, it issubstantially more difficult for a majority of typical feeds to removemore than about percent of the sulfur without excessive hydrocracking.Excessive hydrocracking is undesirable since it unnecessarily consumeshydrogen and produces undesired light products and coke on thedesulfurization catalyst. Furthermore, the nature of the resultingproduct can be altered to the extent that it is not a heavy fuel oilwhereas heavy oils generally have a higher heating value than lighteroils.

Alternatives to attempted in-depth desulfurization of such asphalticfractions have included blending of the I asphaltic hydrocarbon oil withessentially sulfur-free middle distillate oils in order to provide a lowsulfur-containing fuel oil. Other proposals have involved hydrofiningasphaltenic oils, under conditions wherein the asphaltic material ishydrocracked to extinction to produce lower boiling hydrocarbonproducts.

It woulde be highly desirable to provide a process for desulfurizingasphalt-containing hydrocarbon oils so as to render them suitable foruse as heavy fuel oils without the need for either blending suchmaterials with lighter oils or converting the asphalt content of suchoils to lower-boiling products of a completely different nature whichwould prematurely deactivate the catalyst.

A relatively simple process for controlling the hydro desulfurization ofan asphaltic oil without subjecting the oil to excessive hydrocrackinghas now been found, which process comprises passing a residual oil, forexample, which oil contains an asphaltic fraction, through ahydrodesulfurization zone in the presence of hydrogen. The asphaltic oilincreases in aromatics content as it passes through thehydrodesulfurization zone due to in situ aromatics production. Theasphaltic oil having a substantially reduced sulfur content is withdrawnfrom the hydrode sulfurization zone at substantially the point at whichthe aromatic content of the oil no longer increases.

As will be hereinafter demonstrated, it has been discovered that as theasphaltic oil passes through the hydrodesulfurization zone, aromaticfragments of the difiiculty desulfurizable asphaltic portion of the oilare separated from the periphery of the complex, asphaltic molecules.These aromatics act as a solvent for the asphaltic material and enablethis material to be desulfurized without its being excessivelyconverted. By controlling the desulfurization process such that theasphaltic oil is hydrodesulfurized in the presence of a continuallyincreasing percentage by weight of aromatics, or at least underconditions such that the percent by weight of aromatics present does notdecrease, excessive hydrocracking of the asphaltic fraction is avoided,while desulfurization of the asphaltic fraction is optimized under theconditions employed.

Thus, for example, a particular fuel oil containing an asphalticfraction may be produced having a sulfur content below one percent byweight by means of the present invention, by subjecting anasphalt-containing or asphaltic, hydrocarbon oil, such as a crude oil orresidual oil (a reduced crude) containing more than about one percentsulfur to a hydrodesulfurization zone in the presence of hydrogen andwithdrawing a first effluent having a reduced sulfur content relative tothe feedstock from the hydrodesulfurization zone when the aromaticcontent of the oil no longer increases. -The efiiuent comprises hydrogensulfide, a light gas fraction, an oil fraction containing aromatics andsaturates, and a higher boiling, asphaltic fraction. The oil fraction isreferred to as light oil" to contrast it with the higher boilingasphaltic fraction. Next, the hydrogen sulfide, the light gas fraction,and a low boiling portion of the light oil fraction are separated fromthe effluent, and the remaining portion of the effluent comprising theasphaltic fraction and a higher boiling portion of the oil fraction maybe passed to a second hydrodesulfurization zone in the presence ofhydrogen. Each hydrodesulfurization zone is provided with ahydrodesulfurization catalyst that is disposed on a non-crackingsupport. An asphaltic hydrocarbon oil is withdrawn from the secondhydrodesulfurization zone which oil provides a heavy, asphaltic fuel oilcontaining less than one percent by weight sulfur. The sulfur content ofthe resulting heavy fuel oil can be reduced to below one percent, forexample, to below 0.5 and 0.3 percent by weight sulfur.

As will be hereinafter described, the asphaltic, hydrocarbon oilcontaining more than about one percent sulfur which is obtained, forexample, from the first desulfurization zone may be passed to a secondhydrodesulfurization zone in the presence of hydrogen and a light oilfraction comprising aromatics and saturates having a boiling point belowthe asphaltic portion of the feedstock to the secondhydrodesulfurization zone, and controlling the amount of light oilpresent in the zone, and thus the amount of aromatics and saturates thatare passed to the hydrodesulfurization zone to increase thedesulfurization rate and permit the recovery of a heavy, asphaltic fueloil containing less than about one percent sulfur.

Thus, the present invention can provide an asphaltcontaining heavy fueloil, which is practically sulfur free, without the need for blending theasphaltic fuel oil with a sulfur-free middle distillate oil in order toobtain an oil having a low sulfur content, although such blending is notprecluded in accordance With the present invention.

The term asphalt or asphaltic as employed in the present specificationis intended to include the resins and asphaltenes present in crude oil.Asphalt can constitute approximately to 30 percent by volume or more ofcrude oil and has an initial boiling point of about 1040 F. It isobtained in refineries by a propane deasphalting process (solventextraction), or from the residues obtained from distillation.Asphaltenes are highly aromatic and consist of large molecules of fusedaromatics rings and normally contain the greatest sulfur concentrationof any constituent of the full range crude. Unlike other crudefractions, asphalt also contains metals, principally nickel andvanadium. Thus, the asphaltenes and resins may be distinguished from theremainder of the crude oil, which material comprises saturates andaromatics, by virtue of the solubility of these aromatics and saturatesin propane and the insolubility of the asphaltenes and resins inpropane.

The propane-soluble aromatics include benzenes, naphthalenes,thiophenes, benzothiophenes, and dibenzothiophenes as the predominantmolecular species, while the saturates include the non-aromatic,propane-soluble species, such as the naphthenes, e.g., cyclohexanes, andthe paraflins, e.g., dodecane, and sulfur-containing compounds, such ass-butyl mercaptan. Thus, the material commonly referred to as asphaltcomprises the residue of a propane extraction. On the other hand, resinsand asphaltenes are themselves separable by a pentane extraction, byvirtue of the fact that asphaltenes are insoluble in pentane, while bothresins and oils are soluble in pentane.

It has been proposed to subject crude oil or a reduced crude containingthe asphaltene fraction to a hydrodesulfurization reaction in order toreduce the sulfur content of the crude. The crude oil or reduced crudeis passed over Group VI and Group VIII metals on a noncracking support,such as alumina, in the presence of hydrogen and the sulfur level isrelatively easily reduced from about 4 to about 1 percent by weight,i.e., a 75 percent sulfur removal. However, after about 75 percentsulfur removal is effected with a particular feed, such as a Kuwaitcrude for example, the sulfur content of the feed suddenly becomes quiterefractory and a great deal of hydrocracking is experienced in order toaccomplish additional sulfur removal thereby consuming a great deal ofhydrogen and changing the nature of the product. Thus, the point atwhich the sulfur remaining in the crude oil becomes refractory may varywith the nature of the particular type of crude oil. This point may beeasily determined experimentally. As previously mentioned, theasphaltenes consist of large molecules of fused aromatic rings andcontain sulfur in the interior of the large molecules, thereby renderingthe sulfur extremely difficult to remove. In addition, the asphaltcontains all of the metals, such as nickel and vanadium that are presentin the crude, and these metals readily deposit on the catalyst tendingto deactivate the catalyst and reduce its effectiveness. For thesereasons, more severe conditions in the form of higher temperatures andpressures are required to remove more than 75 percent of the sulfur fromthe asphaltic oil and such severe conditions result in hydrocracking,i.e., a severing of the carboncarbon bonds of the asphaltene moleculeresulting in lower molecular weight materials, rather than desulfurization by splitting carbon-sulfur bonds.

In order to more fully understand the process of the present invention,reference is made to the drawings wherein:

FIG. 1 is a flow diagram illustrating the desulfurization of anasphaltic, reduced crude in two stages;

FIG. 2 illustrates graphically the percent yield of materials boilingabove the initial boiling point of the feed to a desulfurization reactoras the average reactor temperature increases;

FIG. 3 graphically illustrates the change in concentration of thearomatic, saturate, asphaltene and resin fractions of anasphalt-containing reduced crude as the degree of desulfurizationincreases;

FIG. 4 graphically illustrates volume percentages of aromatics andsaturates that are removed at various interstages flash points; and

FIGS. 5 to 9 are diagrammatic schemes for obtaining a low sulfur, lightaromatic-rich fraction from one portion of the feedstock to solubilizeand reduce the viscosity of the solution of asphaltenes and resinspresent in the residual portion of the feed prior to desulfurization ofthe asphaltenes and resins.

- Referring to FIG. 1, a reduced crude such as a 50 percent reducedKuwait crude which contains the entire asphalt content of the full crudeand therefore also contains all of the nickel and vanadium and mostrefractory sulfur content of the full crude is charged to the processthrough line 10 and is pumped through line 14, preheater 16, line 18,solids filter 20 and line 22 and drum 24. From drum 24. is the liquidoil charge is passed through line 24 to feed pump 30.

Liquid from pump 30 is admixed with hydrogen from line 32 and passedthrough line 34, valve 36, line 38 and furnace 40. Liquid flow valve 36is disposed in a nonfully preheated liquid hydrocarbon line. Recycledhydrogen along with make-up hydrogen (if desired) are introduced intothe liquid charge to the reactor prior to the preheating thereof.Recycled hydrogen is passed through line 42 and valve 44, While make-uphydrogen may be charged through line 46, compressor 48 and valve 50. Therecycled hydrogen and any makeup hydrogen are introduced to therelatively cool liquid charge through line 32.

A preheated mixture of liquid charge and hydrogen in line 54 may bepassed through a guard reactor (not shown), if desired. An efiluentstream from the guard reactor is charged to the main reactor 60containing catalyst beds 62, 64 and 66. This stream may have a 650 F.+boiling range, for example.

The hydrodesulfurization catalyst employed in the process of the presentinvention is conventional and comprises, for example, Group VI and GroupVIII metals on a non-cracking support. Thus, the catalyst may comprisenickel-cobalt-molybdenum or cobalt-molybdenum on an alumina support. Thealumina may be stabilized with 1 to 5 percent by weight of silica. Thepreferred catalyst is a nickel-cobalt-molybdenum on alumina containingless than 1 percent silica which catalyst may or may not be sulfided.Magnesia is also a non-cracking support. An especially preferredcatalyst comprises a particulate catalyst comprising particles betweenabout and 4 inch in diameter, such as described in US. Pat. 3,562,800 toCarlson et al., which patent is hereby incorporated by reference. Thesame or a different hydrodesulfurization catalyst can be employed ineach stage.

An essential feature of the present invention is that the catalyst isprovided on a non-cracking support, since the process of the presentinvention is essentially a noncracking process in that very littlematerial is produced having a boiling point below the initial boilingpoint of the feed. However, high boiling material in the feed may becracked to produce lower boiling products still in the boiling range ofthe feed. Thus, whereas prior processes have employed techniquesinvolving, for example, high silica-containing catalysts, e.g. percentor more silica, to hydrocrack the asphaltenes in the feedstock, theprocess of the present invention involves mainly the severance ofcarbon-sulfur bonds of the asphaltenes and resins in order todesulfurize the difficulty desulfurizable asphalt, rather than crackingcarbon-carbon bonds, which results in lower molecular weight materials.Of course, sulfur is removed from oils during the present process, butthis type of sulfur removal is relatively easy to accomplish. In otherwords, the cracking of carbon-carbon bonds as in prior processesemploying cracking produces a material other than a heavy fuel oil andis outside the scope of the present invention. Thus, a relatively minoramount of material is produced having a boiling point below the initialboiling point of the feed to the hydrodesulfurization unit.

As previously mentioned, hydrogen is introduced along with the feedstockby means of line 32. Conventional reaction conditions in thehydrodesulfurization reactor are employed, for example, a hydrogenpartial pressure of 1000 to 5000 pounds per square inch, preferably 1000to 3000 pounds per square inch, is employed. A hydrogen partial pressureof 1500 to 2500 pounds per square inch is especially preferred. The gascirculation rate may be between about 200 and 20,000 standard cubic feetper barrel, generally, or preferably about 3000 to 10,000 standard cubicfeet per barrel of feed, and preferably containing 85 percent or more ofhydrogen. The mol ratio of hydrogen to oil may be between about 8:1 and80:1.

It is an essential feature of the present invention that a hydrogenpartial pressure of at least 1000 pounds per square inch is provided.This is necessary in order to desulfurize the asphaltic oil of thepresent invention to the necessary extent and in order to get thehydrogen to the reactive surface of the asphaltene molecule. It is thechemical activity as expressed by the partial pressure of hydrogenrather than total reactor pressure which determines hydrodesulfurizationactivity.

Hydrodesulfurization reactor temperatures may range between about 650and about 900 F, and preferably between about 680 and 800 F.

Referring again to FIG. 1, each succeeding catalyst bed 62, 64 and 66,may have a larger volume than the bed just prior to it. If desired,there may be 4 to 6 catalyst beds in the reactor and, if desired, eachreactor bed may have 25 percent, 50 percent, 100 percent or morecatalyst than the bed just prior to it.

As previously mentioned, because there is no guard chamber line 54 willlead directly to the reactor. The eflluent in line 54 is joined by ahydrogen stream from line 67, valve 69, line 70 and valve 72 so that ahydrocarbon and hydrogen stream is charged to the top of the reactorthrough line 74. As previously mentioned, the reactor stream is passedthrough catalyst bed 62 and due to the exothermic nature of thehydrodesulfurization reaction, it becomes heated in passagetherethrough. Temperatures between the various catalyst beds may becontrolled by employing a hydrogen quench which may be introduced bymeans of line 76, valve 78, sparger 80, and line 82, valve 84 andsparger 86. Finally, the reaction mixture passes through catalyst bed 66and then leaves the reactor with, for example, about a one percent byweight sulfur content.

Referring now to FIG. 2, it is seen that as an asphaltic hydrocarboncrude oil residue containing 4 percent sulfur is passed through ahydrodesulfurization zone, such as reactor 60 in FIG. 1, to produce a 1percent sulfur-containing 660 F.+ product, Well over percent of theproduct boils at 660 F. or above (the initial boiling point of the feed)as the average reactor temperature is increased to compensate forcatalyst aging. However, when the reaction temperature is raised to 800F., excessive hydrocracking to lower boiling materials results. Thiscauses a drop in the curve as to the materials boiling below the initialboiling point of the feed. At this point excessive hydrocracking occursin order to produce a 1 percent sulfur-containing 660 F product.

As will be hereinafter demonstrated, the process of the presentinvention permits the efiicient removal of greater than 75 percentsulfur from an asphaltic oil and overcomes the refractory nature of suchoils.

Reference is now made, once again to FIG. 1. Next, the eflluent in line88 is passed to a high pressure flash chamber 90 wherein lighthydrocarbon gases, hydrogen sulfide, hydrogen and a controlled portionof the relatively highly desulfurized saturates and aromatics areremoved by means of line 92. The operation of this interstage flashchamber 90' constitutes a critical feature of the present invention. Bycarefully selecting the flash temperature at the process pressure, theproper amount of light oil, which is to be removed from the feedstock tothe second hydrodesulfurization feed stream, is determined. The reasonfor the removal of an amount of light oil will be demonstrated byreference to FIG. 3.

Referring now to FIG. 3, the composition of a 650 F.+ hydrocarbon oilcontaining various proportions of aromatics, saturates, resins andasphaltenes is presented, as the oil is passed through ahydrodesulfurization reactor, such as reactor 60 of FIG. 1. As shown inFIG. 3, the resins and asphaltenes content of the feed steadilydecreases with increasing sulfur removal due to the severing ofcarbon-sulfur bonds thereby breaking off molecular fragments. Theaccumulation of these molecular fragments is reflected in the build-upof lower molecular saturates and aromatics and particularly aromatics.This increase in aromatics content in the liquid is beneficial becausethe aromatics constitute a solvent for the highly viscous resins andasphaltenes, whereas the resins and asphaltenes are not solvated bysaturates. The deulfurization of each fraction continues until about 75percent sulfur removal, and at that point the resins and aromaticscurves reach a plateau which indicates no further cracking of fragmentstherefrom. At the same time, the total aromatics and saturates contentdoes not increase further, but an increase in saturates level isaccompanied by a decrease in aromatics level. This indicates that at 75percent sulfur removal the aromatics tend to become saturated, whichrepresents not only a fruitless consumption of hydrogen, but deprivesthe remaining resins and asphaltenes of aromatic solvent, while merelyincreasing the amount of saturates, which is a mere dispersant.

At the 75 percent sulfur-removal level, the sulfur becomes refractory tofurther desulfurization so that further desulfurization is accompaniedby a loss in aromatics and a sharp increase in saturates. Both of thesefactors are detrimental in regard to removing further sulfur andovercoming the refractory nature of the oil at this point, since at the75 percent sulfur-removal level most of the unremoved sulfur isconcentrated in the resins and asphaltenes. Thus, a loss of aromaticsdeprives the viscous resins and asphaltenes of some solvation, while theformation of saturates imparts an excessive dispersion to the systemtending to excessively dilute the remaining sulfur and thereby lower thereaction rate.

Thus, while operating the first hydrodesulfurization reactor, thearomatic content of the oil can be measured as it passes through thereactor, and the oil should be withdrawn from the reactor when thearomatic content of the oil is no longer increased. This was thesituation at 75 percent desulfurization in FIG. 3. The aromatic contentcan increase 25 to 40 percent by weight or more as it passes through thereactor. Even a small increase in aromatics concentration is beneficial,e.g. 2 to 5 or percent by weight. 'Of course, the aromaticsconcentration should not increase to such a great extent that it undulydilutes the sulfur which is to be removed.

Without attempting to limit the present invention to any particulartheory or mechanism, it is believed that up to the 75 percent sulfurremoval there is a removal of fringe sulfur from the complexheterocyclic ring structure of the asphaltene molecule accompanied by anin situ production of aromatic hydrocarbons mainly due to a severing ofthe carbon-sulfur bonds on the fringe aromatic rings. This production ofaromatics is important, since asphaltene molecules have a tendency toform colloidal crystallites or aggregates, if the molecules are poorlysolubilized. Thus, up to the 75 percent sulfur removal level, there issuflicient in situ production of aromatics to solubilize the asphalteneparticle and permit contact of the internal, heterocyclic sulfur withthe hydrogen and catalyst necessary to desulfurize the asphaltene.However, at the 75 percent desulfurization level, the ratio of aromaticsto saturates is sufliciently small that there is insufficient solventand too much diluent to effect the removal of the refractory sulfur.

For this reason, it is necessary to control the aromatic content of theasphaltic feed to the second hydrodesulfurization zone. This may beaccomplished by flashing or otherwise separating, e.g. by distillationor flashing followed by partial condensation of flashed hydrocarbon andrecycle, a controlled portion of the normally liquid eflluent oil fromthe first desulfurization zone to provide the proper aromatics contentto the stream.

Thus, it is essential to include sufficient aromatics in the feed to thesecond desulfurization reactor to solubilize the resins and asphaltenesin the oil and to deagglomerate any asphaltene aggregate which may beformed. At any given flash temperature, the removal of a particularquantity of aromatic hydrocarbons normally removes an even greaterquantity of saturated hydrocarbons. With certain crudes the flash mayremove more aromatics than saturates, in which case the benefit of theflash will be derived from removal of excess dispersent. A compromisemust be reached wherein sufflcient aromatics are charged along with thefeed to the second hydrodesulfurization stage so that the resins andasphaltenes are adequately solvated in order to permit properdesulfurization of the resins and asphaltenes, without the quantity ofaromatics being so high that the total aromatics and saturates fed tothe second stage will excessively dilute and disperse the sulfur to beremoved and thereby reduce the reaction rate. The volume of aromaticsand saturates, respectively, which may be removed at a given flash pointis shown in FIG. 4.

Referring now to the solid lines of FIG. 4, an analysis of the saturateand aromatic content of the first sulfur removal stage effluent is shownfor various flash points when measured at atmospheric pressure. Thus, ifthe interstage flash point is 500 F, all of the aromatics boiling above500 F. will enter the second stage and will be available for solvatingthe resins and asphaltenes. However, at 500 F., the stream comprisesabout 81 percent by volume saturates and about 19 percent by volumearomatics. This amount of aromatics together with the even greateramount of saturates which necessarily accompany it, might excessivelydilute the asphaltic compounds and thereby lower the reaction rate. Onthe other hand, if the interstage flash point is as high as 800 F., theoil fed to the second stage will have a higher ratio of aromatics tosaturates which is desirable in order to accomplish high solvation withminimum diluent. However, at the 800 F. fiashpoint, the total amount ofaromatics which enter the second stage may not be great enough todissolve the resins and asphaltenes and to sufliciently lower theirviscosity to permit desulfurization. Accordingly, a proper balance ofaromatics to saturates must be employed in order to obtain optimumdesulfurization. This point may be easily experimentally determined fora particular stream undergoing desulfurization. For example, employingas a feed the residue represented by the overhead designated by thesolid lines of FIG. 4, the best results are obtained by flashing at thetemperature which gives about a 650 F. separation regardless of theelevated pressure of the flash. This provides not only suflicient totalamount of aromatics in the stream, but a ratio of about 72 percentsaturates by volume to about 26 percent aromatics in the overhead. Aswill be hereinafter demonstrated, the employment of an excessive amountof aromatics in the desulfurization reaction can be as detrimental tosulfur removal as employing too little aromatics.

The dashed lines of FIG. 4 designate another possible overhead saturatesand aromatics distribution in which aromatics begin to predominate in a550 F. overhead fraction. However, since a 550 F. flash includes all thelighter material, the total overhead will still predominate in saturatesover aromatics.

Referring again to FIG. 1, a predetermined amount of liquid is flashedand removed by means of line 92. Depending upon the nature of thefeedstock employed and the conditions utilized in the desulfurizationreaction, the flashed material in line 92 may contain, for example,between about 5 and about 60 percent by weight of the liquid effluentfrom the desulfurization reactor 60, preferably between about 10 andabout 35 percent by weight. Suitable flash temperatures include, forexample, between about 500 and about 800 F., preferably between about600 and about 700 F. (These temperatures refer to atmospheric pressureand of course will be different at the process pressure.) An especiallypreferred flash point for the interstage flash is 650 F. The mostdesirable amount of liquid to be flashed or otherwise separated from thedesulfurization eflluent stream may be easily determined experimentally.The flashed material in line 92 is passed to a high pressure flashchamber unit 94 wherein hydrogen, hydrogen sulfide and light hydrocarbongases are separated from a liquid hydrocarbon fraction. The gases arewithdrawn by means of line 96 and are subjected to purification andseparation, including various scrubbing operations and the like, inrecycle gas recovery unit 97. Substantially all of the hydrogen sulfidethat is produced in reactor 60 is recovered by means of line 93.Hydrogen now free from hydrogen sulfide and light hydrocarbon gases isrecovered from unit 97 and is passed by means of line 99 to recycle gascompressor 101 and recycled for utilization in the process by means ofline 103. The nonflashed liquid fraction is discharged from flash unit94 by means of line 98.

The bottoms fraction from the flash unit is discharged by means of line100 and is admixed with makeup hydrogen, which is provided by means ofline 102,

valve 105 and line 107. In addition, recycle hydrogen may be added tothe make-up hydrogen in line 107 from line 103 by means of line 109,valve 115 and line 113. H desired, make-up hydrogen may be passed fromline 103 directly to line 108 by appropriate valving (not shown). Thecombined stream may be charged to a furnace 106 in order to raise thetemperature of this stream if desired. However, furnace 106 is optional,as the stream 104 may be already at the desired desulfurizationtemperature for introduction by means of line 108 to the secondhydrodesulfurization reactor 110.

The temperatures and pressures employed in reactor 110 may be the sameas those described for the hydrodesulfurization reactor 60. Likewise,the desulfurization catalyst which is employed in reactor 110 may beidentical to that described previously for reactor 60. It is noted atthis point that the hydrodesulfurization catalyst is even more activefor removal of nickel and vanadium than it is for removal of sulfur.Most of these metals will be removed in the first hydrodesulfurizationreactor 60. The heaviest laydown of such metals is at the inlet toreactor 60. The second desulfurization reactor 110 will act as a metalsclean-up stage, and the catalyst therein will not collect as much metalsas does the catalyst in reactor 60. Hence, the first stage catalyst willremove most of the metals and will become deactivated by metals muchfaster than the second stage catalyst.

The desulfurized eflluent from reactor 110 is discharged by means ofline 114 and is passed to a flash unit 116 for removal of light gasesincluding hydrogen, hydrogen sulfide and light hydrocarbons. Thisgaseous stream is sent by means of line 118 to high pressure flash unit94. Meanwhile, a bottoms fraction including the asphaltic product streamof the present invention is passed by means of line 120 to distillationcolumn 122. A sulfur containing stream comprising sour gas and sourwater is removed from column 122 by means of line 124. This stream ispassed to a gas treatment plant (by a means not shown) to recover sulfurtherefrom. A naphtha fraction is withdrawn from the column 122 by meansof line 132. This naphtha stream may be employed as a wash liquid forthe separation of the light hydrocarbons from the hydrogen in line 96(by a means not shown). A furnace oil or heavier fraction may bewithdrawn through line 133 and be employed in a manner hereinafterdescribed to provide additional aromatics to the second stagedesulfurization reactor 110. A product stream 134 is discharged from thedistillation column 122. This desulfurized oil stream containsasphaltenes and resins, and is especially useful, without furtherblending, as a fuel oil, particularly since less than one percent byweight sulfur is contained therein. Thus, this heavy, asphaltic fuel oilcontains, for example, between about 0.3 and about 0.5 percent by weightsulfur or less, which is well within the requirements of even thestrictest ordinances for sulfur content of heavy fuel oils.

The process of the present invention provides a percent yield of notless than 40 or 50 and up to 80 or 90 percent by Weight of materialhaving a boiling point greater than the initial boiling point (I.B.P.)of the feed to the first desulfurization reactor. Therefore, very littlehydrocracking occurs in accordance with the present invention and thehydrogen consumption will be generally in the range of only 150 to 1500and preferably in the range of 300 to 1000 standard cubic feet perbarrel of feed. The feed to the hydrodesulfurization reactor can have anI.B.P. of not less than 375 F., and will preferably have an I.B.P. of atleast 620 or 650 F.

Thus, the amount of material obtained from the secondhydrodesulfurization zone whose boiling point is lower than 375 F., or650 P. will not exceed 50 or 60 percent,

generally, or preferably or 20 weight percent. A feed having an I.B.P.greater than 650 F., e.g. vacuum tower bottoms having an I.B.P. in therange of 750 F. to 900 F. or more may be employed in the process of thepresent invention. In that event, the amount of material obtained fromthe second hydrodesulfurization zone whose boiling point is below 650 F.will not exceed 10 or 20 weight percent. Accordingly, the process ofthis invention may be characterized as an essentially non-crackingprocess."

Likewise, the fuel oil product from each hydrodesulfurization unit has atotal asphaltenes plus resins content of at least 10 or 20 and can have30 or 40 up to percent by weight of that present in the feed to thefirst hydrodesulfurization unit. The fuel oil product from the secondhydrodesulfurization unit has a preferred total content of resins plusasphaltenes of at least 40, 50 or 70 up to percent by weight of thatpresent in the feed to the second hydrodesulfurization zone. This is afurther indication that the resins, and particularly the asphaltenes maybe desulfurized without their complete destruction, as was previouslyproposed.

A modification of the process shown in FIG. 1 is illustrated in FIG. 5,which is a simplified schematic diagram.

Referring now to FIG. 5, a reduced crude oil is introduced by means ofline 220 to an atmospheric distillation unit 222 wherein a lightasphalt-free distillate fraction having a 630650 F. end point (E.P.) isWithdrawn by means of line 224, while a heavy asphalt-containing 630-650 F.+ fraction containing 4 percent sulfur is discharged by means ofline 226 from the distillation column 222. The asphalt-containingbottoms fraction 226 is passed to a vacuum distillation unit 228 whereinadditional light oil is discharged by means of line 230 and is admixedwith the lighter fraction in line 224 and introduced along with hydrogenfrom line 233 into a hydrodesulfurization zone 232 by means of the line234. Zone 232 may be operated with a conventional gas oildesulfurization catalyst at a temperature, for example, in the range ofbetween about 400 and about 800 F., but at a lower hydrogen partialpressure (e.g., below 1000 p.s.i.), than is employed for thedesulfurization of an asphaltic oil. The asphalt-free distillate iseasily completely desulfurized in the zone 232 and is discharged bymeans of line 236 and passed to distillation unit 238 from which anoverhead fraction containing hydrogen, hydrogen sulfide and light gasesis removed by line 240 and processed as previously described to recoverhydrogen and light hydrocarbons.

An aromatic-rich, furnace oil and higher fraction is discharged from thedistillation unit 238 by means of line 244. Meanwhile, anasphalt-containing oil -is withdrawn from the vacuum distillation unit228 by means of a line 246. This stream may have, for example, aninitial boiling point of about 1000 F. and contain about 5.5 percent byweight sulfur. The stream 246 is passed through a blending zone 250wherein the asphalt-containing stream is admixed with a controlledportion of the aromatic-rich fraction from the line 244 in order toobtain the desired viscosity and solvency for the asphaltenes and resinscontained in the stream. Then hydrogen is added through line 248. Theheaviest product from distillation unit 242 can be blended with theproduct 268, if desired.

Next, the asphaltic fraction containing solubilized resins andasphaltenes is passed by means of line 252 to a firsthydrodesulfurization zone 254 and subjected to desulfurization under theconditions previously described in regard to the reactor 60 of FIG. 1.The eflluent from zone 254 has a reduced sulfur content and is passed bymeans of line 256 to an interstage flash unit 258 wherein hydrogen,hydrogen sulfide, light hydrocarbon gases, and a controlled portion ofaromatics and saturates is discharged by means of line 260. Aspreviously discussed, a flash point is selected so as to optimize theamount of aromatic solvent available for solubilizing the asphaltenesand resins in the feed to the second hydrodesulfurization zone. Anefiiuent stream 262 is discharged from the flash unit 258 and is admixedwith hydrodesulfurization unit 266 wherein the sulfur content of theasphaltic oil is reduced to below one percent by weight. The heavy oilproduct stream is discharged from the second hydrodesulfurization unit266, which unit is operated in the manner described for unit 110 in FIG.1, and is withdrawn by means of the line 268 and treated as previouslydescribed for separation of hydrogen sulfide, light gases and the like.

Thus, the system of FIG. provides a parallel mode of operation whereinan initially-separated lighter portion of the crude is desulfurized andis utilized to pro vide the desired viscosity and solvency fordesulfurization of the heavy, asphaltic portion of the crude oil.

Another modification of the FIG. 1 process is illustrated in FIG. 6.Referring now to FIG. 6, an asphaltic oil is introduced by means of line320 to a distillation unit 322 for separation into an aromatic-poorfraction on which is withdrawn from unit 322 by means of line 324 and anaromatic-rich fraction containing 4 percent sulfur. An aromatic-richasphaltic oil is discharged from unit 322 by means of a line 326. Forexample, distillation unit 322 may be operated to provide an asphalticfraction having an initial boiling point of about 650 F.

The asphaltic stream in line 326 is admixed with hydrogen which isintroduced by means of line 328 and the combined stream is passed bymeans of line 330 into a hydrodesulfurization unit 332 which is operatedin the manner previously described. The effiuent from zone 332 iswithdrawn by means of line 334 and is introduced into a high pressureflash unit 336 where, for example, material boiling below 800 F. isdischarged by means of line 338. Accordingly, an asphaltic oil having aboiling point of 800 F.+ is discharged from the flash unit 336 by meansof the line 340.

Referring momentarily to FIG. 4, it is seen that at an 800 F. flashpoint there is a relatively high aromatic to saturate ratio. However,the total aromatic content of the oil at this point may be less thandesired. Accordingly, a controlled amount of an aromatic-rich fractionis introduced into the stream in line 340 by means of line 342 in orderto provide the resins and asphaltenes with a proper degree of solvency.The combined stream is then admixed with hydrogen, which is introducedby means of line 344 and introduced into hydrodesulfurization reactor346 where the sulfur level of the asphaltic fuel oil is reduced to belowone percent by weight.

The eflluent from zone 346 is removed by means of line 348 and isintroduced into distillation unit 350 where an aromatic-rich fraction isseparated and recovered by means of line 352. A controlled portion ofthe material in stream 352 is recycled by means of line 342 foradmixture with the asphaltic stream in line 340 as previously described.The light gases are discharged from distillation unit 350 by means ofline 354, While a substantially sulfurfree, asphaltic, heavy fuel oil isrecovered from line 356. Substantially all of the asphaltenes and resinsfed to distillation unit 350 are recovered in line 356 with theasphaltic fuel oil. The recycle stream 342 is devoid of asphaltenes. Theasphaltenes are not recycled to the first hydrodesulfurization zone,since they would deactivate the catalyst prematurely. They are notrecycled to the second hydrodesulfurization zone, since they havealready been desulfurized, and such recycle would serve no usefulpurpose.

In the foregoing manner depicted in FIG. 6, a light, desulfurized,aromatics-rich solvent for the asphaltic material is obtained from theproduct stream.

Still another modification of the present invention is shown in FIG. 7,wherein an asphaltic feed stream is introduced by means of a line 420 toa distillation unit 422 to reduce the feed and prepare ahydrodesulfurization feed for passage through line 426. The asphalticfraction is discharged by means of the line 426 from unit 422 and isadmixed with a controlled portion of an aromatic-rich stream which isintroduced by means of line 428. The stream in line 428 can be anaromatic-rich fraction boiling within the range of between about 400 andabout 1050 F., preferably between about 650 F. and about 900 F. Thecombined stream is admixed with hydrogen, which is introduced by meansof line 430, and a stream having a boiling point of about 650 F.+ andcontaining about 4 percent by weight sulfur is introduced by means ofline 432 into hydrodesulfurization zone 434. This unit may be operatedat a temperature of 690790 or 800 F. An eflluent stream having a sulfurcontent of about one percent by weight sulfur is introduced by means ofline 436 into flash unit 438. A light oil and gas fraction containingsubstantially all of the hydrogen sulfide produced is flashed from theunit 438 and discharged by means of the line 440, which stream may have,for example, a 650 F. E.P.

An asphaltic stream having a boiling point of, for example, 650 F.+ isadmixed with hydrogen which is introduced by means of line 442 andintroduced by means of line 444 into second hydrodesulfurization zone446, which may be also operated at about 690 to 790 or 800 F. Theeffluent from the second desulfurization zone 446 has a sulfur contentof less than one percent and is passed by means of line 448 todistillation unit 450. An aromatics-rich, asphaltene-free fraction iswithdrawn from unit 450 by means of line 452 and is recycled by means ofline 428 for admixture to the asphaltic oil feedstock to the firstdesulfurization zone 434. Hydrogen sulfide and light gases are withdrawnfrom the distillation unit 450 by means of line 454, while a low sulfurasphaltic fuel oil is recovered by means of line 456. Excess from stream452 not recycled can be blended with product in line 456 to reduce thesulfur content of the product.

Thus, the arrangement of FIG. 7 utilized a low sulfur aromatic-richproduct stream for solubilizing the asphaltenes and resins in a heavydesulfurization feedstock having a relatively high I.B.P.

A modification of the system of FIG. 7 is shown in FIG. 8.

The process of FIG. 8 is similar to FIG. 7, however, as shown in FIG. 8,the eflluent from flash unit 438 that is withdrawn by means of line 440is passed to a flash unit which is provided with cooling coils. Hydrogensulfide and light gases are withdrawn by means of line 443. Theremainingheavier eflluent is withdrawn from the flash unit 441 by means of theline 445 and is passed by means of pump 447 for admixture with stream444 for introduction into the second stage desulfurization unit 446.

The mode of operation of FIG. 8 permits the employment of a flashtemperature for unit 438, which temperature may be the same as thatemployed in the hydrodesulfurization units 434 and 446. At the same timethe lower temperature flash unit 441 which is provided with coolingcoils permits the separation of hydrogen sulfide and light gases and thereintroduction of a material having the optimum aromatics content andinitial boiling point. l

Thus, for example, if it were determined that the optimum interstageflash temperature corresponded to 650 F. at atmospheric pressure and thedesulfurization units 434 and 446 are being operated at about 700 F.,the flash unit 438 may also be operated at 700 F. However, the lowertemperature flash unit 441 is operated at 650 F. and thus permits thereturn by means of the 650 F.+ material by means of line 449.

Thus, the modification of FIG. 8 avoids the need for reducing thetemperature of the stream in line 436 and the reheating of stream 444.

Referring now to FIG. 9, an asphaltic, sulfur-containing, hydrocarbonoil is introduced by means of line 520 to distillation unit 522 forseparation of the feed into light gases, which are withdrawn by means ofline 524, and an aromatic-rich fraction, which is discharged by line526, which is to be employed in a manner hereinafter described. Thisaromatic-rich fraction may have a 650 F. E.P.

An asphaltic bottoms fraction having an initial boiling respectively,while the resins and asphaltenes have lost point of, for example, about650 F. is discharged by sulfur to the least extent, viz., 2.37 and 4.95percent by means of line 528 and is admixed with hydrogen from weightsulfur, respectively. Finally, even after the second line 530 prior tointroduction into first desulfurization stage desulfurization, the onlysignificant sulfur remaining zone 532. The desulfurized effluent fromzone 532 is inis present in the resins and asphaltenes. troduced bymeans of line 534 into flash unit 536. As be- In addition, 1t 1s seenfrom Table I, that the aromatlc fore, the optimum flash point has beenpreviously detercontent of the feed has gone from 55.45 weight percentto mined and a light oil fraction containing saturates and 60.45 Weightpercent after the first stage flashing, and finalaromatics is flashedoff along with light gases and hydro- 1y up to 61 91 weight percentaromatics in the product. At gen lfid b means f lin 538, the same t1methe weight who of aromatlcs to resins plus Th asphaltic il i passed f thfl h nit 536 nd asphaltenes increases from about 2 to 1 to about 4 to 1.mixed with hydrogen from line 540 and introduced by Thus, Table Iclearly illustrates the criticality Of providing means of line 542 tothe second stage desulfurization zone Sufficient aromatics in theasphaltic stream in order to 546, I ddi i h h l i feed to h Zone 546 isolubilize the resins and asphaltenes and to deagglomerate admixed withthe aromatic-rich stream 526. Thus, the y asphaltene aggregates so as toPermit desulfurization distillation operation unit 522 is conductedunder condit Likewise, this table Shows the eiitieaiitY of avoid tionsso that the stream 526 has a selected boiling range g an excess ofliquid including aromatics, especially and aromatics content whichprovides maximum sol vation e i j beyond What is required to contributea 80infor the asphaltenes present in the feed to the second dehlhllngeffect, Since Such an excess of low-Sulfur liquid sulfurization zone546. The effluent from zone 546 is disy t to i p and dilute t esulfur-containing. charged by means of the line 548 and is passed to 11mresins, resms and asphaltenes and dlminish their chance lation unit 550for separation of the asphaltic fuel oil proof Contact With the catalystIt Should be further noted duced therefrom by means of line 552. Lightgas stream t t the fnomatie Pontent of the teed t0 the Second Stage 554and naphtha stream 556 are recovered and treated as 15 rlchet 1naromatics, 60-45 Weight Percent than is the usuaL feed to the firststage, i.e., 55.45 weight percent. This is The following examples arepresented to further illusdue, In P to the age flashing Which retrate hi i moves saturates in much greater proportion relative to EXAMPLE 1 thearomatics present in the flashed, light oil stream. In

each stage, the weight ratio of aromatics to resins plus asphaltenes toaccomplish solvation should be at least 1 to 1 and is preferably 1.5 or2 to 1, and can be 4 or 5 to 1. The aromatics can be present in thefeed, can be introduced by recycle or can be produced in situ.

It is further interesting to note that Table I reveals that thesaturates, which are flashed 01f, are the most highly desulfurizedfraction and therefore have the least need for passage through thesecond stage of desulfurization.

The following example illustrates the refractory nature of an asphalticfeedstock when it is attempted to remove more than one percent sulfurwithout employing the process of the present invention.

An asphalt-containing, reduced crude oil containing about 4.09 weightpercent sulfur and hydrogen are intro- 30 duced into ahydrodesulfurization zone containing a nickel-cobalt-molybdenum catalystdisposed on a noncracking, alumina support. The hydrodesulfurizationoperation is conducted at temperatures of about 650-820 F. and ahydrogen partial pressure of 2000 p.s.i.g., and the resulting asphalticmaterial is flashed at a temperature corresponding to 650 F. at oneatmosphere so as to optimize the amount of aromatics and saturatespresent in the efiiuent. The 650 F.+ asphaltic fraction that iswithdrawn from the flashing unit has a sulfur content of about 1.09weight percent sulfur and is introduced into a secondhydrodesulfurization zone where the desulfurization is EXAMPLE 2 alsoconducted at about 650 F. to 820 F. while employ- A 22 percent reducedKuwait crude containing an asing the same catalyst that is employed inthe first stage. phalt fraction and 5.43 percent by Weight sulfur issub- A heavy fuel oil is obtained having a sulfur content of jected todesulfurization. The initial boiling point of the 0.58 weight percentsulfur. The distribution of sulfur in crude is 556 and the boiling rangeextends to 1400 F.+. each of the various fractions of the oil undergoingde- After the sulfur content is reduced to 4.77, the I.B.P. is

sulfurization is set forth in Table I, below: 514 and the boiling rangeextends to 1400 F.+.

TABLE I Feed to first HDS Feed to second HDS Zone zone Fuel oil productSulfur in Sulfur in Sulfur in Fraction traction Fraction fractionFraction fraction (percent (percent (percent (percent (percent (percentby wt.) by wt.) by wt.) by wt.) by wt.) by wt.)

Saturatcs 17. 9s 3. 42 22. 24 0. so 22. 34 0. 49 Aromatics 55.45 5. 0460. 45 1.12 61.91 0.56 Resins 16.73 5. 59 13.76 2. 31 12. 72 1. 56Asphaltcnes- 9. 84 6. 99 3. 4. 95 3. O3 3. l3

As seen from Table I, it is apparent that in an asphaltic Asdesulfurization continues, and the sulfur content is feed containing atotal of about 4.09 weight percent sulreduced to 1.41, the I.B.P. is 509with the boiling range fur, the sulfur content of such feed isrelatively evenly extending to 1400" F.+. However, when the sulfurcondistributed between the saturates, aromatics, resins and tent isreduced to 0.83, the I.B.P. drops 90 F. to 466 asphaltenes. However,after the feed is passed through the F. with the boiling range extendingto l400 F.+.

first desulfurization zone and the 650 F. E.P. fraction is The resultsof this run are shown in Table H, below:

TABLE II Reduction in sulfur content during Feedstock desulfurizationSulfur, percent by wt 5. 48 4. 77 1. 41 0.80 Boiling range, F 566-1,400+ 514-1,400+ 509-1,400| 466-1,400+ Desulfun'zation, percent 12. 2 74.0 85. 3 API 6. 5 8. 7 17. 7 20. 3

removed, i.e., at 1.09 weight percent total sulfur, the satu- Theforegoing data shows that the sulfur content of the rates and aromaticshave lost sulfur to the greatest extent, feed becomes very refractory atabout 74 percent deviz., they are down to 0.80 and 1.12 weight percentsulfur sulfurization. At 12 percent sulfur removal the I.B.P. is

Feedstock Composition change during (percent desulfurization (percent bywt.) by wt.)

Desulmrizatlon, percent 12. 2 74 85 saturates-.. 11 16. 7 26. 1 33. 2Aromatics 39 41. 2 56. 6 49. 9 Resins 82 25. 7 15. 1 15. 3 Asphaltenes18 16. 4 2. 2 1. 6

The foregoing data show that as the depth of sulfur removal increases,resins and asphaltenes diminish and are converted to saturates andaromatics.

The following Table IV gives a molecular weight comparison of thearomatics, saturates and residuals during the course of desulfurization.

TABLE IV Feed- Molecular weight change stock during desulfurizationDesulfurization, percent 12.2 74 85 saturates, M.W- 430. 400 410Aromatics, M.W 490. 0 530 400 Total Residuals, M.W 1,080 590. 0 490 420The data in Table IV show that as the depth of desulfurizationincreases, the molecular weight of the total residuals decrease, but notsignificantly below the molecular weight of the initially formedsaturates and aromatics. This molecular weight pattern furtherillustrates that during the breaking of carbon-sulfur bonds in theresins and asphaltenes, hydrocarbon fragments are produced which areabout in the same molecular weight range as are the feed saturates andaromatics.

EXAMPLE 3 In order to illustrate the effect of both dilution andconcentration of the resins and asphaltenes in the second stage feedupon the second stage desulfurization rate, a 650 F. flash temperatureis employed on a feedstock that had been subjected to a single stage ofhydrodesulfurization. The asphaltic, flash residue is subjected to asecond stage hydrodesulfurization, and it exhibits a desulfurizationreaction rate constant of about 85. The desulfurization reaction rateconstant is calculated in a conventional manner, i.e.

( Lb. oil lb. oil 1b. sulfur hr.-lb. catalyst This reaction rateconstant can also be expressed as 1 1 R-( LHSV where S is pounds ofsulfur per pound of oil in the product; S is pounds of sulfur per poundof oil in the feed; and LHSV is volume of oil per hour per volume ofcatalyst.

For comparative purposes, a desulfurized furnace oil having a boilingpoint in the range of 400 to 650 F. and containing 0.07 weight percentsulfur is added to the residue of the 650 F. flash operation. Thefurnace oil is comprised of about one-half saturates and one-halfaromatics. Upon passing this asphaltic feed through a second stagedesulfurization, the desulfurization reaction rate drops to 75. Thus,excessive dilution of the feed to the second stage actually reduces thedesulfurization reaction rate, even though aromatics are being added.

For further comparison, 30 percent by weight of a light portion of thesecond desulfurization stage feed is removed making the feed equivalentto about the residue of an 800 F. flash. In this instance, the reactionrate for desulfurization in the second stage falls to 40 therebyillustrating the effect of the inadequate solvation for the resins andasphaltenes. It is clearly evident that the amount of saturates plusaromatics which accompanies the resins and asphaltenes into the secondstage has a distinct effect upon the second stage reaction rate, andthat too much diluent can have a detrimental effect upon desulfurizationreaction rate, just as does too little aromatic solvent.

EXAMPLE 4 In order to determine the effect of adequate, as contrasted toexcessive solubilization of an asphalt-containing feed, a residual,high-boiling feed having an initial boiling point of about 800 F. ischarged to a hydrodesulfurization process, and 76.2 percent by weightdesulfurization is accomplished. Next, a second sample of the feed isdiluted with 30 volume percent of a lower boiling gas oil which had beenpreviously desulfurized to the extent of to percent by weight. Theaddition of the gas oil, which comprises a high proportion of aromatics,increases the desulfurization to 80.3 percent by weight.

For comparative purposes, a further sample of the feed is diluted with40 percent by volume of a gas oil and is subjected to desulfurization asbefore. In this instance, there is a loss in desulfurization activityand the desulfurization drops to 76.3 percent by weight. The employmentof 64 percent by volume of gas oil reduces the degree of desulfurizationeven lower to 69.4 percent by weight. The results of these tests areillustrated in Table V below:

TABLE V Effect of gas oil dilution Gas Oil, percent by vol 0 30.0 40. 064. 0 Desulfurization, percent by wt"..- 76. 2 80. 3 76. 3 69. 4Vanadium removal, percent by wt 83. 5 78. 6 74. 6 77. 8

EXAMPLE 5 A test is performed utilizing in the first and second stages aone-thirty-second inch nickel-cobalt-molybdenum on alumina catalyst. Inthe first stage the catalyst exhibits a six month life with astart-of-run temperature of 690 F. and an end-of-run temperature ofabout 790 F. in hydrodesulfurizing a Kuwait reduced crude from 4 to 1weight percent sulfur at a LHSV of about 0.8. The catalyst life in thesecond stage is even longer. The one weight percent sulfur eflluent fromthe first stage is flashed to remove 650 F. end point material based onatmospheric pressure and is charged to the second stage with hydrogen toreduce the sulfur level to 0.5 weight percent. The second stagestart-of-run temperature is 690 F. and the temperature at days is only763 F. The test can continue until the temperature is 790 F. Therefore,the first and second stages of this invention can operate for 3, 4, 5, 6or even 7, 8 or 12 months at a LHSV generally ranging between 0.1 and 10or preferably between 0.3 and 1 7 1.25. The life of the second stagecatalyst is longer than the life of the first stage catalyst.

Although the invention has been described in considerable detail withparticular reference to certain preferred embodiments thereof,variations and modifications can be effected within the spirit and scopeof the invention as described hereinbefore, and as defined in theappended claims.

We claim:

1. A process for the hydrodesulfurization at a hydrogen partial pressurefrom 1,000 up to 5,000 psi. of a sulfurcontaining heavy feed oilcontaining high boiling asphaltenes and resins and lower boilingsaturates and aromatics to produce a product of reduced sulfur content,said process comprising charging said feed oil downfiow in a firsthydrodesulfurization zone in the presence of hydrogen in a reaction zonecontaining catalyst particles to inch in diameter at a temperature inthe range between 680 to 800 F., said temperature being increased withinsaid range to compensate for loss in hydrodesulfurization rate due tocatalyst aging, said catalyst comprising both Group VI and Group VIIImetal on alumina, operating said first zone at a liquid hourly spacevelocity between 0.1 and 10, the catalyst and process conditions providing at least about 75 percent sulfur removed and in situ productionof aromatics in said first zone to increase the aromatics concentrationin the oil at least 5 weight percent in said first zone, withdrawing anefiiuent from said first zone containing asphaltenes in the presence ofsaid increased aromatics concentration, flashing said efiluent toseparate hydrogen, hydrogen sulfide, light hydrocarbon gases and alow-boiling liquid fraction containing a major amount of saturates and aminor amount of aromatics from a flash residue having a further increasein the concentration of aromatics relative to said efliuent, chargingsaid flash residue to a second hydrodesulfurization zone and continuinghydrodesulfurization of the oil in said second zone.

2. The process of claim 1 wherein the aromatics concentration in the oilincreases at least 10 weight percent in said first zone.

3. The process of claim 1 wherein the aromatics concentration in the oilincreases at least 25 weight percent in said first zone.

4. The process of claim 1 wherein the aromatics concentration increases25 to weight percent in said first zone.

5. In the process of claim 1 recovering an eflluent from the second zoneof which not more than 20 weight percent has a boiling point lower than375 F.

6. In the process of claim 1 recovering an eflluent from the second zoneof which not more than 10 weight percent has a boiling point lower than375 F.

7. The process of claim 1 which produces an oil wherein at least weightpercent boils above the initial boiling point of the feed oil.

8. The process of claim 1 wherein the catalyst contains nickel, cobaltand molybdenum.

9. The process of claim 1 wherein the catalyst age ex tends to at leastfive months of operation.

References Cited UNITED STATES PATENTS 3,392,112 7/1968 Bercik et a1.208216 3,563,886 2/1971 Carlson et a1. 208210 3,481,860 12/1969 Borst,Jr. 208213 3,617,525 11/1971 Moritz et al 208211 2,723,943 11/1955McAfee 208213 3,483,118 12/ 1969 Gleim et al 208209 3,649,526 3/1972Pollitzer 208210 3,429,801 2/ 1969 Gleim et a1 208210 3,714,031 1/ 1973Van der Toorn et a1. 208216 DELBERT E. GANTZ, Primary Examiner G. I.CRASANAKIS, Assistant Examiner

